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832

Appendix II.

the element coils, rises, and passes into the receiver, beneath the
baffle plate which separates the hot steam from the colder feed.
The circulation is therefore caused by difference of density. K is
a mud collector, or blow-off chamber. It is usual to make steam
at a much higher pressure than is required, and let it pass through
a reducing valve on its way to the cylinder; this gives drier
steam by throttling, ensures a more perfect deposition of lime
and magnesia, gives a steadier pressure at the engine, and permits
the use of a smaller boiler for a given power. (See p. 919.)

In the Thorny croft boiler, Fig. 801, there is one steam receiver

,

II

A, but two mud drums B and c, and circulation is due to a rise in
the tubes D D and a fall in the downcomers F, outside the casing.
The feed enters at G, the water being delivered along the receiver
bottom, while two principal features are the entry of the water
tubes into the steam space of the receiver, and their tortuous*
form for the purpose of meeting the draught normally* H is a
baffle plate, and j the steam pipe.

The   Yarrow   and   Normand  boilers   are   similar  to   the
Thornycroft, but the first has straight tubes, while those of the
second are but slightly bent; in both cases they enter the
drum at the bottom.   The DM Temple is of the same type,   The

^ Lagrafal d'Allest, Orwlle, and De Naeger are similar to